For elucidation of structure and function relationships in human leukocyte interferon AlphaA and AlphaD, a number of peptides from the C-terminal region were synthesized by conventional solution synthesis and solid phase synthesis and their biological and immunological characteristics were investigated. The peptide 121-166 was shown to contain antigenic determinants and used to produce antibodies specific for native leukocyte interferon. Analysis using smaller peptides resulted in localization of two distinct determinants in this sequence. Assays of antiviral activity and cellular membrane receptor binding are in process. Attempts to develop further the methodology of enzymic peptide synthesis showed that trypsin - in the presence of methylamine - catalysed formation of peptide bonds of nonspecific amino acid residues such as glycine, alanine, serine, valine and histidine. Further, we have introduced two novel Alpha-amino protecting groups compatible with enzymic coupling in aqueous phase: the acid labile citra-conyl group and the phenylacetyl group which can be cleaved enzymatically with penicillinacylase. These new techniques will eventually be applied to the synthesis of other highly pure peptides of biological interest.